The invention relates to a tie rod (tie anchor) for strip-type tension members used in the building trade, especially fiber-reinforced plastic lamellae having at least one anchoring body positively connected to the tension member by means of adhesion and/or friction whereby said anchoring body can be supported on a fixed abutment.
It is known in the art to attach pretensioned strip-type tension members on the outside of the supporting framework after erection to increase the load capacity (strengthening) or to restore the original load capacity (reconstruction) of supporting frameworks made of reinforced concrete or prestressed concrete. Fiber-reinforced plastic lamellae are preferably used for this purpose aside of steel lamellae (bands), especially synthetic materials reinforced with carbon fibers, synthetic materials reinforced with aramide, and synthetic materials reinforced with glass fibers.
A significant characteristic of these fiber-reinforced synthetic materials, in particular the preferably employed synthetic materials reinforced with carbon fibers, is the fact that the strip-type tension members made from these materials show linear elastic behavior up to the breaking point. Attention must be given in the necessary anchoring of the end pieces of the tension members to maintain single-axis tensile stress conditions. A dual-axis tensile stress condition caused by substantial stress spikes at the clamping point and/or at the point of deflection would lead to damage or even to destruction of the strip-type tension member.
The transition point from the free span length of the tension member to the anchoring zone is non-uniform in terms of stiffness, specifically at the adhesive attachment of the strip-type tension member to the anchoring bodies short and which absorbs the load initiated by the tension member through shearing stress whereby said spike in shearing stress exceeds the locally admissible shearing stress in the adhesive joint and reaches the ultimate stress (breaking stress). The crucial breaking criteria in case of the use of an adhesive is hereby the exceeding of cohesion of the adhesive and/or the breaking of the plastic matrix of the strip-type tension member. The thereby formed breaking shear-stress front moves along the adhesive joint until the adhesive connection breaks down completely.
It is know from prior art (DE 198 49 605 A1 and corresponding to U.S. Pat. No. 6,584,738) to apply an additional clamping force between the anchoring body and the tension member glued thereto to increase the adhesive effect. The thereby developing dual-axis stress condition (longitudinal stress/limited transverse pressure) is harmless for the tension member since no transverse stress occurs. There occurs rather an increase of the crucial breaking strength. However, the spike in shearing stress is thereby not decreased at the transition from the free span length to the anchoring zone.
For the solution of the problem of decreasing or of avoiding a spike in shearing stress at the transition from the free span length into the anchoring zone, it has been proposed in prior art to alter the adhesive characteristics along the force introduction area in such a manner whereby a relatively soft adhesive is used at the transition to the anchoring (less shear modulus) and the adhesive characteristics on the other end of the anchoring is altered in such a way that the adhesive is provided with a high shear modulus and whereby the adhesive acts substantially stiffer. However, the selection of adhesive material and especially the maintaining of set conditions in the application of the adhesive demand very high requirements and they are not controllable, especially after application.
It is also known from prior art to embed a perforated metal plate or similar material in the adhesive joint. A generally lower shear modulus of the adhesive joint is achieved thereby without reducing the total load capacity. The damaging spike in shearing stress can, nevertheless, be reduced thereby—but not to a sufficient degree in many cases of application.
It is therefore the object of the invention to design a tie rod of the aforementioned type in such a manner that the development of a spike in shearing stress is avoided which locally exceeds the ultimate stress in the adhesive joint or in the region of friction.